Background: The angle formed by the intersection of the line between triradiate cartilage of both hips (Hilgenreiner's line) and a line drawn between triradiate cartilage and the external roof of the acetabulum is called the acetabular index (AI), the angle formed by line of proximal femoral physis and middiaphysis axis of femur is called the epiphyseal angle (EA). Both decrease with age, but it is not described whether there is a relation between them in this development. It would be very useful to compare its evolution in children with normal hips, in order to compare them with dysplasic hips in the same groups of age.
Objective: To study the radiographic angles and the relationship between the acetabulum and the proximal femur.
Materials and methods: Retrospective study was performed on pelvic X-rays in 224 children between 1 and 3 years of age. Children with injuries, sepsis or inflammatory disease were excluded. We measured by goniometer (error +/-1 degrees ): the AI, and the angle of the proximal femoral physis (EA). We have compared statistically the parameters.
Results: The AI was 21.1 degrees (+/-3.8), 19.9 degrees (+/-3.5), and 16.1 degrees (+/-4.2) and EA was 75.8 degrees (+/-5.1), 75.9 degrees (+/-6.3), and 75.6 degrees (+/-4.7), at the ages of 1, 2, and 3 years, respectively. A significant difference was noted for AI between 2 and 3 years old (P = 0.003), but there was not significative EA decrease. No significant correlation was found between AI and EA among the different groups, nor overall (r = 0.03). Sex or side was not a significant factor for both angles.
Conclusion: The AI decreases significantly between 1 and 3 years of age, but not the EA. No significant relation was found between them in the different age groups. Normal children's hips have a tendency to spontaneously improve its acetabular morphology during the first 3 years of life.